delete platform cursor before Application
We want the platform cursor to have roughly the same lifespan as the application.
By using QObject parent mechanism, this gets deleted after the Application destructor in the QObject destructor.
This causes an issue that removing an event filter (used by the X11 cursor) calls into the application singleton which is no longer valid.
Tested on X11, still have a cursor, kwin_x11 --replace 5 times in a row didn't crash, whereas it did before
Edited by David Edmundson